Our invention relates to improvements in the diffusers used with gas turbine engines, particularly high performance engines as are employed in the propulsion of aircraft. In such engines, the diffuser and associated compressor are essential components for pressurizing air as a preliminary step in the generation of a high energy, hot gas stream.
Rotary type compressors are often used for this purpose and comprise an impeller, or rotor, which imparts energy to the air, primarily in the form of increased velocity. The high velocities of the air discharged from the exit side of the impeller are too great for practical utilization in supporting combustion of fuel. Therefore, it is accepted practice to provide a diffuser immediately downstream of the impeller. The diffuser decelerates the discharge air to relatively low velocities and converts a major portion of the velocity energy to static pressure energy. In most compressors, the impeller, or rotor, has projecting blades over which the air flows in discrete paths as it is accelerated thereby. Likewise, the diffuser, or stator, has vanes which split the high velocity discharge air into discrete flow paths.
A major problem in the operation of compressors is the phenomenon known as surge. When this condition occurs, flow of air through the compressor is throttled, either locally or completely, and in some cases reverse air flow can occur. The result of compressor surge is a reduction in power in all cases and frequently a flameout of the combustor, in which case there is a complete loss of power.
Surge will occur, at a given engine speed, when the aerodynamic loading on the blades or vanes exceed a given limit, causing separation of the air from the flow passageway surfaces and a condition of high turbulence. This limit varies between different compressor designs and is established for each compressor design by way of what is known as a compressor map. Knowing the characteristics of a given design, it is then possible to control the operation of the engine, primarily through the rate of fuel flow to the combustor, so that there is a margin of safety in both steady state and transient operation.
Several different approaches have been used to solve the surge problem. Conrad in German Pat. No. 1,938,132 and British Pat. No. 1,043,168 show implementations wherein pressure is bled from a higher to a lower level to prevent build up of shockwaves in the diffuser passageway throats. The pressure bleed off is achieved by means of connecting pipes which either recirculate the fluid to a lower pressure point in the system or vent it.
O'Connor in U.S. Pat. No. 3,768,919 shows a pipe diffuser with an aerodynamically variable throat area. A series of ports are provided in the throat region of the diffuser passages to momentarily inject pressurized diffuser exit air to aerodynamically vary the throat flow characteristics and prevent surge during operation of the stage above its normal surge line.
Sobey in U.S. Pat. No. 3,006,145 shows an antisurge control system which makes use of a compressor bleed system. He uses a bleed valve which is responsive to both compressor rotor speed and acceleration of the compressor rotor.
Our invention differs from the above in that we provide slots in the sidewalls of the throat section of each vane of the diffuser. These slots communicate through cavities in each vane with a closed manifold. The benefits achieved by the use of a closed manifold have been verified by means of test instrumentation Data taken from operating diffusers shows that shock waves tend to build up in the throat areas of some passageways before they do in others. This may be due to imperfections in the vanes or can be caused by the shadow effects of strut vanes in the compressor stages. Use of a closed manifold in communication with slots in the passageway walls alleviated the problem in that tendencies for pressure surges in one or more passageways was quickly equalized across all passages through flow into and out of the connecting manifold. This phenomenon was never mentioned in any of the cited patents.